Nilmini,
New Caledonia has been a French Overseas Territory (TOM) since 1946. It was
photogrammetrically mapped in 1954 and again in 1987. At those times the
coordinate systems were based on the International ellipsoid, UTM Grid,
since then collectively called IGN 72. They are now on the ITRF system
using the GRS 1980 ellipsoid (same as WGS 84), and they are still on UTM,
albeit on the new ellipsoid. The new system is referred to as IGN 91 after
the French Institut Geographique National Mission in 1991. Specifically
this is the official system on "Grande Terre de Nouvelle-Caledonie."
The existing paper maps are based on initial points surveyed by astronomical
methods. The resultant is a local "Datum." Although generally reliable
within itself, a local Datum is usually some tens or hundreds of meters
different from a global system that you can access with a GPS receiver.
This is true of existing topographic paper maps of every country in the
world. In a few decades, many of the countries of the world will have
changed over to new Datums that "match" the coordinate system of the GPS
satellites. Right now, it's a bit of a problem and the French have
determined the parameters to relate the old Datum of New Caledonia to the
GPS satellite system called the "ITRF" - International Terrestrial Reference
Frame. Different dates are used with ITRF that mean a lot to a geodesist,
but are meaningless to a cartographer. (Cartographers rarely worry about
millimeter-to-centimeter differences in coordinates).
To transform from local coordinates in New Caledonia, you MUST use a
Seven-Parameter Bursa-Wolf Datum shift where: dX = -97.333 m, dY = +262.726
m, dZ = -310.147 m, Rx = +1.629325", Ry = -0.866434", Rz = -3.146066", dS
= -13.271857 X 10^ -6. Note that you cannot truncate parameters and only
use, say, the first three terms. If you do such a thing, you will NOT
obtain correct transformation results because the translation terms are
figured out AFTER the rotation terms are computed at the center of the
earth.
For details on this sort of thing, note that I publish a monthly column in
"Photogrammetric Engineering and Remote Sensing," and many past columns
(different country each month) are available from the following web site of
the American Society for Photogrammetry and Remote Sensing:
http://www.asprs.org/resources.html
Look under "Grids and Datums." If you like, consider joining the society -
it is the premier international journal on GIS, Photogrammetry, and Remote
Sensing. ASPRS was founded in 1934, and all papers are published in
English.
Clifford J. Mugnier ([EMAIL PROTECTED])
Surveying, Geodesy, & Photogrammetry
Dept. of Civil & Environmental Engineering
LOUISIANA STATE UNIVERSITY
12408 CEBA Building
Baton Rouge, Louisiana 70803
Voice: (225) 388-8536
Facsimilie (225) 388-8652
======================================
----- Original Message -----
From: "Nilmini Wijayakoon" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, 26 May, 2000 1:25 AM
Subject: GEN - Projections for New Caledonia
>
>
> Does anyone have any information about projections used for mapping areas
> in
> New Caledonia? I have some data I have brought into ArcView 3.2 and I
> believe it is UTM zone 58 but don't know much else about projections of
> this
> area. In my search for information, I have come across references to IGN
> 1954, IGN 1987, WSG ...but am not sure what exactly this means. Basically,
> what projection would be used by a GIS person in New Caledonia for their
> data? Also regarding New Caledonia...I have GPS data I have brought into
> ArcView and have been informed I must correct Easting and Northings by 300
> meters (339 and 335 to be exact). Is this a result of the surveyors
> themselves and their equipment or a function of the location itself?
>
> Thanks for any insight provided.
> Nilmini
> ________________________________________________________________________
> Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com
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